The present invention relates to a magnetic field-enhanced etch reactor suitable for both plasma etching and reactive ion etching (RIE) mode plasma etching and, to associated processes for etching semiconductor, conductor and dielectric materials.
Over the last several years, the trend to ever greater device densities and smaller minimum feature sizes and smaller separations in integrated circuits has caused the IC fabrication technology to shift from wet chemical etching to dry or plasma etching technology.
The art includes at least three types of plasma etching systems. FIG. 1 depicts a parallel plate plasma, multiple wafer chemical etching system 10, which includes a closed reaction chamber 11 with a connection 12 to a vacuum pump for partially evacuating the interior of the chamber, and a gas supply 13 for communicating the reactive gas to the chamber through a valve-controlled conduit arrangement 14. The system 10 also includes an energy source 16 which supplies RF energy to a cathode structure 17 and utilizes a grounded anode 18. The wafers 19 are mounted on the grounded anode 18 which extends in a parallel plate configuration relative to the cathode 17. The connection to the vacuum pump is configured to draw the reactive gases into the region between the anode 18 and the cathode 17 for confining the reactive gas plasma formed by the RF energy supplied to the cathode 17.
FIG. 2 depicts a parallel plate reactive ion etching mode, plasma etching system 20, which also includes a substantially closed reaction chamber 21 with a connection 22 to a vacuum pump for partially evacuating the interior of the chamber, a gas supply 23 for communicating the reactive gas to the chamber through a valve-controlled conduit arrangement 24, an RF power supply 26 which supplies RF energy to a cathode structure 27 and a grounded anode 28. In contrast to plasma system 10, FIG. 1, in the reactive ion etching system 20, the wafers 19 are mounted on the cathode 27, which is shielded from and separated from the anode 28.
FIG. 3 schematically illustrates another RIE mode etching system, 30, which, like reactors 10 and 20, is available commercially from Applied Materials, Inc. of Santa Clara, Calif. System 30 includes a cylindrical reaction chamber 31, a hexagonal cathode 37 connected to an RF supply 36, and an exhaust port 32 that is connected to a vacuum pump. The walls of the reaction chamber 31 and the base plate 38 form the grounded anode of the system. Gas supply 33 is communicated into the chamber 31 through port 34 and conduit 35 to a gas distribution ring 41 at the top of the chamber.
The parallel plate plasma system 10 is a relatively high pressure system, operating in the pressure range 100 millitorr to several torr, and thus involves a substantial flow rate of reactive gases into the system. In contrast, the reactive ion etching systems 20 and 30 are operated at low pressures in the range of 1 to 100 millitorr and, thus, use substantially lower gas flow rates. In the reactive ion etching systems 20 and 30, activated ion species in the neighborhood of the cathode have high inherent directionality normal to the cathode and the wafers mounted thereon. By using high frequency RF energy at fairly substantial power levels, the etch rates are increased in systems 20 and 30, despite the relatively low concentration of activated species, because the momentum of the ions bombarding exposed material regions on the wafer surface enhances the chemical reaction between the activated species and the material to be etched. Also, the highly directional mechanical ion bombardment etch component dominates the more isotropic chemical component and imparts high anisotropy to the etching characteristics of the system. Consequently, RIE mode systems such as 20 are preferred for etching very small features such as grooves and trenches in VLSI and ULSI circuits.
The following are important factors and requirements in the design and selection of commercially useful RIE mode etching reactors. First, to provide acceptable device manufacturing yields, the RIE mode reactor must meet certain process requirements such as directionality, selectivity, uniformity, throughput, low particulate levels, etc. Secondly, it is desirable that the RIE mode etch reactor require little or no maintenance, for example, by incorporating in-situ, self-cleaning capability. Other desirable characteristics include the adaptability to factory and reactor automation, small reactor size and low manufacturing cost.
The latter group of factors would tend to favor the use of single wafer systems over batch-type systems, other factors being equal. Moreover, single wafer systems are more convenient for process development (only one expensive wafer is used for each process run) and do not present within-batch, wafer-to-wafer uniformity problems.
However, the single wafer RIE systems typically must be operated at high pressures (&gt;200 mT) in both the plasma mode and in the RIE mode, to increase the etch rate and throughput. Unfortunately, high pressure operation decreases directionality and selectivity and makes it difficult to meet the exacting requirements of VLSI and ULSI device manufacturing. Thus, as is usually true, other things are not equal and in order to obtain commercially viable high throughput as well as acceptable directionality and selectivity characteristics, most RIE mode etchers, including etchers 20 and 30 shown in FIGS. 2 and 3, are low pressure batch-type reactors.
Referring to FIG. 4, there is shown a single wafer RIE mode etch reactor 40 which is an exception to the above-described state of the art. The magnetic field-enhanced, RIE mode plasma etching system 40 is that described in co-pending, commonly assigned U.S. Pat. No. 4,668,338, entitled "Magnetron-Enhanced Plasma Etching Process", issued May 26, 1987, in the name of inventors Dan Maydan et al. The system 40 is a modification of the magnetic field-enhanced CVD deposition system disclosed in co-pending, commonly assigned U.S. Pat. No. 4,668,365, entitled "Apparatus and Method for Magnetron-Enhanced Plasma-Assisted Chemical Vapor Deposition", also issued May 26, 1987, in the name of inventors Foster et al. The U.S. Pat. No. 4,668,338 and U.S. Pat. No. 4,668,365 patents are hereby incorporated by reference. The RIE mode etch reactor 40 uses magnetic field-enhanced etching to provide a relatively high etch rate despite the use of relatively low pressure and, therefore, can provide high throughput without sacrificing directionality and selectivity, or vice versa. The RIE mode etch reactor 40 also decreases the inherent etch non-uniformity which results from interaction between the magnetic field and the plasma in magnetic-field enhanced RIE systems.
The system 40 includes a cylindrical stainless steel vacuum chamber 43. A flanged cathode assembly 42 is mounted within the chamber 43 on insulating posts (not shown). Typically, the cathode 42 is polygonal and has non-magnetic reflector end sections 44A (FIG. 5) formed of conductive non-magnetic material such as aluminum. Outer end sections 44B are formed of material such as Maycor.RTM. insulating material. Power is supplied for plasma operation by an RF system 46, typically a 13.6 MHz system, that includes an RF power supply and a load matching network and is connected to the cathode 42. Reactant gas is communicated to the interior of chamber 43 by one or more inlet tubes or ring manifolds 47 from a gas supply system 48, of gas storage tanks/reservoirs 49-49.
Semiconductor wafers 55 are held by means 51, such as posts or clips, at the side of the cathode. As indicated by arrows 52, the reactant gas flows across the substrate surface, and then via one or more exhaust outlets 53 to a mechanical pump (not shown) via a vacuum valve and Roots blower.
Electromagnets 54 and 56, typically formed of copper coils, are circumferentially positioned about the chamber 43 near the top and bottom thereof. The electromagnets form north and south poles which are reversible by reversing the coil current.
Referring further to FIG. 4, during RIE mode plasma etching operation of the reactor system 40, a selected etching gas or mixture is inlet from the gas supply through the inlet tubes 52-52 to the reaction chamber 43, which is evacuated by the exhaust pump system. As shown in FIG. 5, application of RF power from the power supply 46 creates a low pressure, reactant gas discharge or plasma 57 of electrons, ions and disassociated species in the vicinity of the semiconductor wafers 55. An electric field E is formed across the plasma shield or dark space directed from the positive potential etching plasma toward the surface 58 of the electrode central section. This field accelerates electrons across the sheath away from the electrode surface and accelerates positive ions across the sheath toward the electrode and the wafer 55 to provide the directional ion bombardment etch component which is characteristic of RIE mode plasma etching.
The reversible magnetic field, B, FIG. 5, is applied to the chamber 43 parallel to the substrates 55 and perpendicular to the electric field, E, to control the characteristics of the etch process. The electrons are confined by the magnetic field lines, which prevent the electrons from moving easily from the cathode face 58 to the anode 43. Also, the magnetic and electric fields impart an E.times.B drift velocity to the electrons so that they tend to drift and to move from point to point along the cathode surface. The electrons are concentrated in a band having a net drift velocity along the cathode and substrate. In conjunction with the end reflectors 44A, the E.times.B drift velocity tends to confine the electrons within the plasma.
As mentioned, there is an inherent uniformity problem due to with the interaction between the magnetic field and the plasma in magnetic-field enhanced reactors. The plasma density is higher downstream in the E.times.B direction, providing a higher etch rate. To illustrate, and referring to the FIG. 5 enlarged partial cross-sectional view of the cathode 42 of etcher 40, FIG. 4, the etching rate is greater at the end or side 58 of the wafer corresponding to the positive side of the magnetic field. Reversing the current through the two cylindrical coils 54 and 56 (FIG. 4) reverses the direction of the magnetic field across the wafer, to B'. This reverses the plasma flow so that the higher etching rate is switched to the opposite end 59 of the wafer. By reversing the magnetic field, the inherent etching non-uniformity is partially compensated in that the etch rate and total etching are averaged across the wafer along the direction of the static field.
Other magnetic field-enhanced RIE etchers attempt to minimize the etch non-uniformity using different techniques. For example, one approach mounts permanent magnets beneath the wafer to provide the magnetic field and mechanically moves these magnets to "smear" the field. This approach does not really solve the non-uniformity problem, has potential mechanical problems and does not provide an adjustable magnetic field strength. A second approach known to us also uses fixed permanent magnets to generate the magnetic field and uses very low pressures to minimize non-uniformity.
To summarize the state of the art, presently, batch-type reactors such as those 10, 20 and 30 described above are used in most commercial plasma etching and RIE mode plasma etching reactors. The batch reactors process a relatively large number of wafers at once and, thus, provide relatively high throughput. However, single-wafer reactors have certain advantages, described above, such as their adaptability to automation, small size, low manufacturing cost, and their lack of wafer-to-wafer within-batch uniformity problems, which make such reactors attractive, particularly for etching large, expensive wafers such as 6 inch and 8 inch diameter wafers. Unfortunately, in the past, inter-related problems with etch rates, throughput, directionality/selectivity and within-wafer uniformity have prevented full utilization of the potential advantages of single wafer etches.